Examples of polar lipids include phospholipids (e.g. phosphatidyl choline, phosphatidyl ethanolamine, phosphatidyl inositol, phosphatidyl serine, phosphatidylglycerol, diphosphatidylglycerols), cephalins, sphingolipids (sphingomyelins and glycosphingolipids), and glycoglycerolipids. Phospholipids are composed of the following major structural units: fatty acids, glycerol, phosphoric acid, amino alcohols, and carbohydrates. They are generally considered to be structural lipids, playing important roles in the structure of the membranes of plants, microbes and animals. Because of their chemical structure, polar lipids exhibit a bipolar nature, exhibiting solubility or partial solubility in both polar and non-polar solvents. The term polar lipid within the present description is not limited to natural polar lipids but also includes chemically modified polar lipids. Although the term oil has various meanings, as used herein, it will refer to the triacylglycerol fraction.
One of the important characteristics of polar lipids, and especially phospholipids, is that they commonly contain polyunsaturated fatty acids (PUFAs: fatty acids with 2 or more unsaturated bonds). In many plant, microbial and animal systems, they are especially enriched in the highly unsaturated fatty acids (HUFAs: fatty acids with 4 or more unsaturated bonds) of the omega-3 and omega-6 series. Although these highly unsaturated fatty acids are considered unstable in triacylglycerol form, they exhibit enhanced stability when incorporated in phospholipids.
The primary sources of commercial PUFA-rich phospholipids are soybeans and canola seeds. These biomaterials do not contain any appreciable amounts of HUFAs unless they have been genetically modified. The phospholipids (commonly called lecithins) are routinely recovered from these oilseeds as a by-product of the vegetable oil extraction process. For example, in the production of soybean or canola oil, the beans (seeds) are first heat-treated and then crushed, ground, and/or flaked, followed by extraction with a non-polar solvent such as hexane. Hexane removes the triacylglycerol-rich fraction from the seeds together with a varying amount of polar lipids (lecithins). The extracted oil is then de-gummed (lecithin removal) either physically or chemically as a part of the normal oil refining process and the precipitated lecithins recovered. This process however has two disadvantages: (1) the seeds must be heat-treated before extraction with hexane, both increasing the processing cost and denaturing the protein fraction, thereby decreasing its value as a by-product; and (2) the use of the non-polar solvents such as hexane also presents toxicity and flammability problems that must be dealt with.
The crude lecithin extracted in the “de-gumming” process can contain up to about 33% oil (triacylglycerols). One preferred method for separating this oil from the crude lecithin is by extraction with acetone. The oil (triacylglycerols) is soluble in acetone and the lecithin is not. The acetone solution is separated from the precipitate (lecithin) by centrifugation and the precipitate dried under first a fluidized bed drier and then a vacuum drying oven to recover the residual acetone as the product is dried. Drying temperatures of 50-70.degree. C. are commonly used. The resulting dried lecithins contain approximately 2-4% by weight of oil (triacylglycerols). Process temperatures above 70.degree. C. can lead to thermal decomposition of the phospholipids. However, even at temperatures below 70.degree. C. the presence of acetone leads to the formation of products that can impair the organoleptic quality of the phospholipids. These by-products can impart musty odors to the product and also a pungent aftertaste.
To avoid use of non-polar solvents such as hexane and avoid the negative side effects of an acetone-based process, numerous processes have also been proposed involving the use of supercritical fluids, especially supercritical CO.sub.2. For example, U.S. Pat. No. 4,367,178 discloses the use of supercritical CO.sub.2 to partially purify crude soy lecithin preparation by removing the oil from the preparation. German Patent Nos. DE-A 30 11 185 and DE-A 32 29 041 disclose methods for de-oiling crude lecithin with supercritical CO.sub.2 and ethane respectively. Other supercritical processes have been proposed which include adding small amounts of hydrocarbons such as propane to the supercritical CO.sub.2 to act as entraining agents. However, supercritical fluid extraction systems are very capital expensive and cannot be operated continuously. Further, extraction times are long and the biomaterials must be dried before extraction, and this increases the difficulties of stabilizing the resulting dry product with antioxidants. All of these factors make the supercritical process one of the most expensive options for extracting and recovering polar-lipid material or mixtures of these materials. As a result, alternative processes using extraction with liquid hydrocarbons at lower pressures have been described. For example U.S. Pat. No. 2,548,434 describes a method for de-oiling oilseed materials and recovering crude lecithin using a liquid hydrocarbon at lower pressures (35-45 bars) but elevated temperatures (79.degree. to 93.degree. C.). U.S. Pat. No. 5,597,602 describes a similar process that operates at even lower pressures and temperatures. However, even with these improvements supercritical fluid extraction remains very expensive and is not currently used to produce phospholipids for food use on a large commercial scale.
The primary commercial source of HUFA-rich polar lipids is egg yolk. Two primary methods are used for the recovery of egg phospholipids on an industrial scale. Both require the drying of the egg yolk before extraction. In the first process the dried egg yolk powder is extracted first with acetone to remove the triacylglycerols. This is then followed by an extraction with pure alcohol to recover the phospholipids. In the second process, pure alcohol is used to extract an oil/lecithin fraction from the dried egg yolk. The oil/lecithin phase is then extracted with acetone to remove the triacylglycerols, leaving behind a lecithin fraction. There are several disadvantages to both of these methods: (1) the egg yolk must first be dried before processing, an expensive step, and additionally this drying process can damage and denature the proteins, severely reducing their value as a food ingredient; (2) the alcohol and acetone concentrations used in these processes must be above 80%, and preferably higher than 90% in concentration, to be effective. Higher purity solvents are more expensive and use of high solvent concentrations leads to denaturation of the proteins, reducing their value; and (3) separate solvent recovery conditions must be available to recover two types of solvents, increasing the cost of equipment. All three of these disadvantages lead to significant increases in the costs of separating and recovering polar lipid-rich fractions from egg yolk.
Canadian Patent No. 1,335,054 describes a process for extracting fresh liquid egg yolk into protein, oil and lecithin fractions by the use of ethanol, elevated temperatures, filtration and low temperature crystallization. The process however has several disadvantages: (1) denaturation of the protein due to the use of high concentrations of ethanol; (2) the process is limited to ethanol; (3) the process removes the proteins first and then the lecithins are recovered from the oil fraction. The purity of the lecithin product is not disclosed.
In light of the current state of the art, there remains a need for an improved extraction technology for food-grade polar lipid products which is less expensive to operate, which protects the value of the associated by-products, and Which protects the overall quality of the HUFAs in the polar lipid products.